Pilot-tone chain for fm stereo receiver



Jan. 10, 1967 F. DIAS ETAL 3,297,826

PILOT-TONE CHAIN FOR FM STEREO RECEIVER Filed Sept. 24, 1964 2 Sheets-Sheet l FIG. 1

0 Di scriminotor Detector Hp qgll k/ i L 8 O 0 0 Q2- T. E 3 INVENTORS ong FZenung D1626 JouKe 77. fypkema Jan. 10, 1967 F. DIAS ETAL 3,297,826

PILOT-TONE CHAIN FOR FM S' I'EREO RECEIVER Filed Sept. 24, 1964 2 Sheets-Sheet 2 FIG. 2

M O -35v INVENTORS FZemirzg 131L616 1701476, 72. Fy Kama United States Patent 3,297,826 PILOT-TONE CHAIN FOR FM STEREO RECEIVER Fleming Dias, Chicago, and Jouke N. Rypkema, Villa Park, Ill., assignors to Zenith Radio Corporation, Chicago, Ill., a corporation of Delaware Filed Sept. 24, 1964, Ser. No. 398,948 3 Claims. (Cl. 179-15) The present invention is directed to improvements in the so-called pilot-tone chain of a frequency modulation receiver for utilizing a stereophonic broadcast signal.

In accordance with the present standards of the Federal Communications Commission, the program signal of a stereophonic frequency modulation broadcast is a carrier that has been subjected to complex modulation. The first term of the modulating function is known as the sum or (A +B) information, wherein A and B represent the two audio signals characteristically employed in stereophonic reproduction. The second term of the modulating function is a subcarrier which has been suppressedcarrier amplitude-modulated by the difference or (A -B) information obtained from these same two audio signals. The final term is known as a pilot. It is a tone at half the frequency of the subcarrier which conveys the difference information.

Since necessary information of the broadcast is transmitted by a suppressed-carrier modulation, it is necessary that the fundamental frequency of the suppressed carrier modulated signal be reconstructed or otherwise be made available at the receiver and this need is filled by the pilot tone. Those portions of the stereophonic receiver which reconstruct the subcarrier to be used as a demodulation signal and which operate on the modulated subcarrier to derive the difference information of the broadcast make up the so-called pilot-tone chain. Illustrative circuit structures of the pilot chain are described and claimed in Patent 3,133,993, issued May 19, 1964 and in Patent 3,129,288 issued Apr. 14, 1964, both of which are assigned to the assignee of the present invention.

The present invention represents an improvement in the pilot chain and has to do most particularly with the net- Work of the indicator that is energized only during stereophonic reception. Where the pilot chain is transistorized, its largest power drain is the indicator circuit and it is found desirable to so arrange the structure that the power drain is essentially uniform whether a stereophonic signal is being reproduced or not. The resulting advantage is that the operating potentials supplied from a common power supply to the various active devices of the chain are then substantially constant and the circuit operation is stabilized.

Accordingly, it is an object of the invention to provide an improved pilot-tone chain for a stereophonic frequency modulation receiver.

It is a particular object of the invention to provide a pilot-tone chain for such a receiver having an indicator light to designate stereophonic reception and characterized by essentially constant operating potentials for the active elements of the chain in the presence or absence of a stereophonic program signal.

A further particular object of the invention is to provide a pilot-tone chain for an FM stereo receiver in which the current drain of the pilot-to-ne-indicator network is substantially the same in the presence or absence of a stereophonic program.

A frequency modulation receiver for utilizing a stereophonic signal which includes a pilot tone of a particular frequency has, in accordance with the invention, a pilottone chain comprising a transistorized detector system responsive to the pilot tone for demodulating the stereophonic signal. There is a control circuit in the detector system for developing a control potential of one value in the presence of the pilot tone and of a different value in the absence of the pilot tone. Additionally, there is a multi-branch indicator network having one branch including a pilot indicator which is de-energized in the absence of the pilot tone and having another branch in parallel with the first-mentioned branch and conducting the major portion of the current in the network in the absence of the pilot tone. The impedance of the aforesaid one branch of the network is proportioned relative to that of the other so that the total current drain in the network is approximately the same with the indicator energized as with it de-energized. Actuating means included in the indicator network are responsive to a control potential of the aforesaid one value for energizing the pilot indicator and there are means for using the control potential developed in the control circuit for operating the aforesaid actuating means.

In one aspect of the invention, there is a common power supply in the pilot-tone chain and the indicator network is the largest drain on that supply. The impedance relations of the parallel branches in the indicator network are preferably related to the end that there is no substantial change in load on the power supply in the presence or absence of a stereophonic program. Accordingly the operating potentials applied throughout the chain, other than to the pilot indicator itself, remain essentially constant.

The foregoing and other objects of the invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which like components parts are designated by the same or similar reference characters and in which:

FIGURE 1 represents a frequency modulation receiver having a pilot-tone chain embodying the subject invention, while FIGURES 2 and 3 show different variations that may be made to the pilot chain.

Except for the pilot-tone chain, the receiver of FIG- URE 1 is similar to that disclosed and claimed in De Vries Patent 3,129,288. Both operate in response to the same type of signals and each is a multi-mode receiver which automatically adjusts itself to accommodate the character of the signal instantaneously received. Details of the signal specifications and the conversion as between operating modes are disclosed in the De Vries patent and need not be repeated.

The receiver includes a radio-frequency amplifier of any desired number of stages and a heterodyning stage or first detector all of which are represented by block 10. The input of this unit connects with an antenna system 11 and its output is coupled to a unit 12 which includes stages of intermediate-frequency amplification and one or more amplitude limiters. Following amplifier and limiter 12 is a frequency modulation detector 13 responsive to the amplitude limited intermediate-frequency signal for developing a composite signal representing the modula- 7 26. The emitter of the transistor is returned to ground through a resistor 27 and the collector circuit comprises a load impedance 28 which is tuned to the pilot tone of a stereophonic broadcast in series with resistors 29 and 30 which lead to a potential supply designated -20 v. There is a voltage divider of resistors 31 and 32 extending from source 20 v. to ground and the base of transistor 25 connects with the junction of these resistors.

Since the difference information of the stereophonic program is conveyed by way of suppressed-carrier amplitude modulation, it is necessary to reconstruct the carrier component at the receiver and, therefore, the detector system of the pilot chain has a signal source which is responsive to the pilot tone of a received stereo program for deriving a demodulation signal. As shown, this source is a frequency doubler comprising a pair of diodes 40, 41 the cathodes of which couple to the opposite terminals of the secondary winding of a transformer 42. The center tap of the secondary winding is grounded and the anodes of the diodes connect to ground through a load resistor 43. Since the demodulation signal source is a frequency doubler of the full-wave rectifier type, the amplitude of the demodulation signal is determined by and varies with the amplitude of the pilot tone as applied to the doubler until limiting occurs in the amplifier that precedes the doubler.

The frequency doubler is driven by the pilot tone through a variable gain transistor amplifier 45 which couples the frequency doubler or demodulation signal source to input amplifier 25. The emitter circuit of amplifier 45 includes a low resistance 46 by-passed at the pilot frequency by a capacitor 47 and coupled to ground through a high impedance shown as a resistor 48. The collector circuit of transistor 45 includes the primary of transformer 42 tuned to the pilot tone by a condenser 49 and connected through a resistor 50 to a source of operating potential designed 20 v. The emitter-base circuit of transistor 45 includes, in addition to resistor 46, a resistor 51 in series with an inductor 52 which is inductively coupled to tunedimpedance 28 of amplifier 25. Actually, coils 28 and 52 constitute primary and secondary windings of a transformer. The junction of resistor 51 with coil 52 is by-passed at the pilot frequency by a capacitor 53. The base of transistor 45 connects to voltage source 20 v. through a high impedance 54.

The operating potentials applied to this transistor and the high impedance in its emitter circuit establish a quiescent operating condition of low gain and low D.C. collector current but the transistor exhibits high gain and high collector current during stereophonic reproduction. This is accomplished by a feedback circuit coupling demodulation signal source 40, 41 with the input of ampli fier 45. The feedback circuit includes impedance 43 across which a potential of negative polarity is developed during operating intervals in which frequency doubler 40, 41 is driven by the pilot tone. The high potential terminal of resistor 43 connects through a resistor 55 to the base of a transistor 56. A shunt capacitor 57, in conjunction with resistor 55, forms a filter network which applies asubstantially D.C. potential from the frequency doubler to the base of transistor 56. The collector and emitter electrodes of transistor 56 are connected directly in shunt to resistor 48 in the emitter circuit of transistor 45. Transistor 56 is normally non-conductive but may be rendered conductive in response to the operation of frequency doubler 40, 41.

The demodulation signal from frequency doubler 40, 41 is used in a suppressed carrier detector employing a single transistor 60. If that transistor be of the symmetrical type, having a base and two other electrodes which serve equally well and in alternation as emitter and collector, the demodulator is then essentially the same as that shown in FIGURE 2 of De Vries Patent 3,129,288. However, the use of a non-symmetrical transistor as a suppressed-carrier detector in a stereophonic 4 FM receiver is described and claimed in copending appli cations, Ser. No. 331,340, filed Dec. 12, 1963, now Patent No. 3,151,218, and Ser. No. 282,325, filed May 22, 1963, now Patent No. 3,151,217, both of which are assigned to the same assignee as the present invention. It is this type detector that is indicated in FIGURE 1.

The collector-emitter circuit comprises load resistors 61, 62 and the secondary winding of a transformer 63. That winding is center tapped and coupled to ground through a resistor 64. The signal output of detector 13 is applied to the base electrode of transistor 60 through input amplifier 25 by way of a connection from a tap of tuned circuit 28 through a coupling capacitor 66. A bias potential is applied to the same electrode through its connection between resistors 67 and 68 which form a voltage divider across potential source 20 v. The detected output from resistor 61 is applied through a resistor 69 and an RC network 70 serving to accomplish de-emphasis to the input of a B amplifier 71 which drives a loudspeaker 72. Similarly, the output taken from resistor 62 is supplied over a resistor 73 and a de-emphasis network 74 to an A amplifier 75 driving a second speaker 76, the speakers, of course, being arranged spacially to establish a stereophonic sound pattern.

As previously indicated, it is necessary to apply a demodulation signal from frequency doubler 40, 41 to the detector 60 and to that end the high potential terminal of resistor 43 of the doubler couples through a resistor to the base electrode of a transistor 81. The emitter of this transistor is grounded through a resistor 82 bypassed at the subcarrier frequency by a capacitor 83 and the collector connects through the tuned primary of transformer 63 and a resistor 79 to potential source -20 v.

There is a multi-branch indicator network having one branch including a pilot indicator or lamp which is de-energized in the absence of the pilot tone of a stereophonic broadcast signal. The branch including lamp 100 may be traced from power supply 35 v., resistor 101 and resistor 102. There is another branch to the indicator network in parallel with lamp 100 and conducting the major portion of the current of the network in the absence of the pilot tone. This branch may be traced from source 35 v., resistor 101, the collector-emitter circuit of a transistor 103 and resistor 102. Actuating means are provided in the indicator network, responsive to a control potential for energizing the pilot indicator. This actuating means is transistor 103 which, when biased to its off condition, causes lamp 100 to be energized. There are means for applying a control potential developed in a control circuit of the pilot chain for actuating transistor 103. The control circuit in which this potential is developed includes a resistor 105 connected in series with resistor 48 across potential source -20 v. During monaural operation in which transistor 56 is cut off, a control voltage of one value, specifically a maximum value, is available at the high potential terminal of resistor 48. Whenever transistor 56 is conductive, however, its collector-emitter circuit shunts resistor 48 and causes the potential available at the free terminal resistor 48 to be reduced to a minimum. It is this control potential that is applied to transistor 103 by way of a resistor 106 to actuate transistor 103.

During monaural reception, the pilot chain is essentially inactive because no pilot tone is being received, and no signal is being developed in frequency doubler 40, 41. Under these conditions, transistor 56 is cut off and a control voltage of maximum amplitude is applied to the base of transistor 103, biasing it to a condition of saturation. Accordingly, the collector-emitter path shunts indicator lamp 100 with a low impedance; the lamp is deenergized and all of the current in the indicator network flows through the branch in shunt to the pilot light.

In this operating condition, the output signal from detector 13 is amplified in transistor 25 and is delivered from the connection extending from the junction of resistors 29 and 30 through a coupling condenser 78 to the tapped secondary of transformer 63. The monaural signal is then applied through resistor 61 and de-emphasis network 70 to amplifier 71 and concurrently flows through resistor 62, de-emphasis network 74 to amplifier 75. Speakers 72 and 76 produce the monaural program.

During stereophonic reception, the output from detector 13 is again applied to transistor 25 and the pilot tone is transferred through variable-gain amplifier 45 to frequency doubler 40, 41. Initially, transistor 45 has a low gain because of the high A.C. impedance in its emitter circuit. As an output is developed in the frequency doubler, a voltage is developed across resistor 43 and applied through filter 55, 57 to the base of transistor 56 causing that transistor to become conductive. As a result, the high emitter impedance of transistor 45 is shunted by the low collector-emitter impedance of transistor 56 and there is a regenerative effect causing transistor 45 to assume a condition of high gain. When it has attained high gain, there is a maximum shunting effect of its emitter high impedance and the control potential applied through resistor 106 to transistor 103 assumes a minimum value. Transistor 103 becomes non-conductive and pilot indicator 100 is energized and remains energized throughout the stereo program.

The demodulation signal from frequency doubler 40, 41 is applied through transistor 81 in push-pull to loads 61, 62 of transistor 60. Concurrently, the output of discriminator 13 is applied from the collector circuit of transistor 25 through capacitor 66 to the base electrode of transistor 60. In response to these signals, detection takes place and the difference information is developed across loads 61, 62.

At the same time, the sum information of the stereo program is delivered over coupling condenser 78 to the tapped secondary of transformer 63 to accomplish matrixing with the difference information to the end that the separated A audio signal, after de-emphasis, is delivered to amplifier 75 for reproduction. The B audio information is similarly de-emphasized and delivered to B amplifier 71 for reproduction.

The potential sources designated -35 v. and -20 v. are, in fact, a single source having a normal voltage of -35 volts. The intermediate value of -20 volts is derived through voltage dropping resistors or dividers (not shown) and is the appropriate operating voltage for the transistors of the chain. In practice, the indicator network itself represents by far the greatest drain on the power supply and the change in drain, when pilot 100 is otf as compared to the drain when it is energized, is relatively small. Accordingly, the potential source for energizing the transistors of the chain has a substantially constant value and the operation of the chain is stabilized. This is achieved by proportioning the impedances of the described branches of the indicator network and, if necessary, resistors may be added in either branch to have no significant change in potential in the pilot chain, as between monaural and stereophonic operation.

In one constructed embodiment of the receiver, the following parameters were employed:

Resistor 48 ohms 10,000 Resistor 101 do 1,000 Resistor 102 do 100 Resistor 106 do 4,700 Transistor 103 Type 2N1303 Power drain:

During monaural mils 35 During stereophonic do 25 The arrangement of FIGURE 2 differs from that of FIGURE 1 in that the collector of transistor 81 is coupled through the tuned primary of transformer 63 and resistor 79 to the high potential terminal of pilot lamp 100. As a consequence, the collector voltage of transistor 81 is inadequate during monaural reproduction to have this transistor energized. The detector system for deriving the difference information is, therefore, disabled and protected against adverse effects of noise.

During stereophonic reproduction, however, when transistor 103 is biased to its off condition, the high potential terminal of pilot raises to a sufiicient value to cause transistor 81 to be conductive as required to accomplish stereophonic reproduction.

In the modification of FIGURE 3, the collector-emitter path of transistor 103 is in series with pilot lamp 100 whereas it is in shunt relation to the pilot in the embodiments of FIGURES 1 and 2. Moreover, in the arrangement of FIGURE 3, the parallel branch of the multibranch indicator network includes a resistor which is in shunt to the series arrangement of pilot 100 and the collector-emitter electrodes of transistor 103. The base of the transistor connects through resistor 106 to resistor 43, the load impedance of frequency doubler 40, 41.

In monaural operation, there is zero potential on the base of transistor 103 and the lamp 100 is de-energized. In the presence of a stereophonic program, the voltage developed at resistor 43, in response to the operation of the frequency doubler, causes transistor 103 to conduct and energize pilot indicator 100.

In all three embodiments, a delay bias potential is developed across resistor 102 which has a desirable stabilizing etfect on the circuit. For the arrangements of FIGURES l and 2, the voltage at the base of transistor 103 is at a value of perhaps -8 during monaural and drops to essentially zero during stereo. The potential across resistor 102 is approximately -l.5 on monaural and about 1 volt during stereo. Because of this bias, which serves as an amplitude delay on transistor 103 during stereo reception, transients that may otherwise apply a potential to the base of transistor 103 are ineifective to cause the transistor to conduct and extinguish pilot light 100 unless they are of sufiicient strength to overcome the bias. In the case of FIGURE 3, the base of transistor 103 is at essentially ground potential during monaural and if the emitter were, likewise, at ground, transients could cause the pilot light to give a false indication of stereo reception. The delay bias developed by resistor 102 minimizes this possibility and tends to prevent false energization of lamp 100. Moreover, it is found that in locations of high ambient temperatures, it is possible to encounter a voltage condition at the base of transistor 103 that might energize the pilot lamp and give a false indication of stereo reception. This, too, is minimized by the delay bias provided by resistor 102.

It should also be pointed out that the circuit configuration of FIGURE 3 may be employed to de-energize the pilot chain in the manner of FIGURE 2 during monaural reception. For example, the collector potential of transistor 103 in FIGURE 3 is at -25 volts during monaural and it changes to 2 volts in stereophonic reception. By returning the emitter of transistor 81 through resistor 82 to this collector, the transistor is cut off or has exceedingly low gain during monaural and is conditioned to exhibit high gain in stereo.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. In a frequency modulation receiver for utilizing a stereophonic signal which includes a pilot tone of a predetermined frequency, a pilot-tone chain comprising:

a transistorized detector system responsive to said pilot tone for demodulating said stereophonic signal;

a control circuit in said detector system for developing a control potential of one value in the presence of said pilot tone and of a different value in the absence of said tone;

a multi-branch indicator network having one branch including a pilot indicator which is de-energized in the absence of said pilot tone and having another branch in parallel with said one branch and conducting the major portion of the current of said network in the absence of said pilot tone, the impedance of said one branch being proportioned relative to that of the other so that the total current drain in said network is approximately the same with said indicator energized as with it de-energized;

actuating means in said indicator network responsive to a supplied control potential of said one value for energizing said pilot indicator;

. and means for utilizing the control potential developed in said control circuit for operating said actuating means.

2. A frequency modulation receiver in accordance with claim 1 including means for applying the potential developed across said pilot indicator, when energized, as an operating potential to said detector system.

3. A frequency modulation receiver in accordance with claim 1 in which said actuating means comprises a triode transistor having a collector-emitter path in series with said pilot indicator and connected along with said pilot in 5 shunt to said other branch of said indicator network;

and further in which said means for utilizing the control potential comprises means for biasing said transistor to saturation in the presence of said pilot tone and to cut off in the absence of said pilot tone. 10

References Cited by the Examiner UNITED STATES PATENTS 3,116,372 12/1963 Wollf 17915 15 3,176,075 3/1965 Bially 17915 OTHER REFERENCES Helber: FM Stereo Multiplex Adaptor, Electronics 7 World, November 1962, p. 43 relied on.

20 DAVID G. REDINBAUGH, Primary Examiner.

ROBERT L. GRIFFIN, Examiner. 

1. IN A FREQUENCY MODULATION RECEIVER FOR UTILIZING A STEREOPHONIC SIGNAL WHICH INCLUDES A PILOT TONE OF A PREDETERMINED FREQUENCY, A PILOT-TONE CHAIN COMPRISING: A TRANSISTORIZED DETECTOR SYSTEM RESPONSIVE TO SAID PILOT TONE FOR DEMODULATING SAID STEREOPHONIC SIGNAL; A CONTROL CIRCUIT IN SAID DETECTOR SYSTEM FOR DEVELOPING A CONTROL POTENTIAL OF ONE VALUE IN THE PRESENCE OF SAID PILOT TONE AND OF A DIFFERENT VALUE IN THE ABSENCE OF SAID TONE; A MULTI-BRANCH INDICATOR NETWORK HAVING ONE BRANCH INCLUDING A PILOT INDICATOR WHICH IS DE-ENERGIZED IN THE ABSENCE OF SAID PILOT TONE AND HAVING ANOTHER BRANCH IN PARALLEL WITH SAID ONE BRANCH AND CONDUCTING THE MAJOR PORTION OF THE CURRENT OF SAID NETWORK IN THE ABSENCE OF SAID PILOT TONE, THE IMPED- 